What happens if they need to employ someone new, taking the number of employees, obviously, to 73091? Would that new person have to buy their $2,794,051 share before they start working? Or is that share simply taken off the 73090 and given to the new guy?

In your hypothetical the workers have collective ownership of the company and would have some kind of democratic or consensus-based decision-making process. They would manage their assets through that process, rather than simply dividing things up into equal shares. If - through whatever decision-making process they happen to have - they decide that they need more workers then they would simply recruit more workers.

Or what if one of the 73090 decides now they're a multimillionaire that they don't ever need to work again and cash their share in? They walk away with $2.7m in their pocket. Where did that money come from? Did the 73089 all have to buy their share off them?

If someone wants to quit then they can quit, and take whatever severance package was decided on by whatever decision-making process the workers happen to have. Collective ownership doesn't mean everyone has an equal slice of the pie that they can just take as they please, it means everyone has a say in what happens with all of it. You have an incredibly individualistic view of the world and its sabotaging your understanding.